Thursday, June 6, 2019
The RF noise bridge
Before people had antenna analysers and other modern test equipment, an item in most experimentally-minded hams' shacks was the RF noise bridge. They allow you to measure real and imaginary impedances at radio frequencies. If you've got a modest junk box they are cheap to build.
So what is a noise bridge? It has several parts. The first is a broadband white noise source. This can simply be a Zener diode with DC applied. The noise level is very low so it needs to be amplified with a two or three transistor amplifier. Output from that is fed to a bridge type circuit. This includes a connection to the unknown load. Other 'arms' of the bridge include a variable resistor and variable capacitor. Both have large knobs with numbers written on them during calibration. These numbers reflect real and imaginary reactance.
Finally, as with any bridge, you need some sort of indicator of output level. Here you don't need an accurate measurement of level as you're only tuning for a null (ie when the signal drops in level and probably disappears). A general coverage radio receiver or transceiver with an S-meter is ideal. If you wish to change the frequency that you wish to do the test just retune the receiver.
Applications include testing and adjusting antennas and antenna coupling units. You could also use it to check toroids for suitability at radio frequencies. That's useful as purchased toroids can be quite expensive and it's a bit of a gamble to used salvaged types without knowing their properties at RF.
The above just scratches the surface on what RF noise bridges are and can do. More information on them is in these articles:
Have you built an RF noise bridge? How did you find it? Please leave your notes in the comments below.
PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.
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